Simply the best answers.

Simulation of high temperature flame, over predicts the temperature than expected, what can be done to get a realistic temperature field?

Global reaction mechanisms with one or two steps inevitably neglect the intermediate species. In high-temperature flames, neglecting these dissociated species may cause the temperature to be over predicted. A more realistic temperature field can be obtained by increasing the specific heat capacity for each species. International Flame Research Foundation (IFRF) has created a set of specific heat polynomials as a function of temperature for different species. These specific heat polynomials can be used to get a more realistic temperature field.
In ANSYS Fluent12, these specific heat polynomials are inbuilt and can be set for the required mixture material using RP variable as follows: